1. Field of the Invention
The present invention relates to a technical field for a hydraulic brake boosting system for boosting the force for depressing a brake pedal of, for example, an automobile, with power realized by hydraulic pressure of operating fluid so that braking force is enlarged. More particularly, the present invention relates to a technical field for a pump operation control apparatus for a hydraulic brake boosting system for controlling the operation of a pump for generating the hydraulic pressure of the operating fluid.
2. Description of the Related Art
Brake boosting systems for vehicles, for example, automobiles, include a hydraulic brake boosting system which incorporates a hydraulic brake boosting apparatus for boosting the force for depressing a brake pedal so as to enlarge the braking force. The hydraulic brake boosting system enables great braking force which cannot be obtained from only the depressing force which is applied to the brake pedal. Moreover, the force for depressing the pedal can be reduced.
An hydraulic brake boosting system of the foregoing type incorporates a pump which is operated by a motor so as to generate hydraulic pressure of the operating fluid and an accumulator for accumulating the generated hydraulic pressure. When the brake pedal has been depressed, the hydraulic pressure accumulated in the accumulator is introduced into the hydraulic brake boosting apparatus. Thus, the hydraulic brake boosting apparatus produces an output obtained by boosting the depressing force applied to the pedal in accordance with the depressing force. The thus-produced output causes a master cylinder to be operated.
The above-mentioned brake boosting system has a necessity that hydraulic pressure of a predetermined level can assuredly be introduced into the hydraulic brake boosting apparatus when the brake is operated. Therefore, the operation of the pump must be controlled in such a manner that predetermined hydraulic pressure is always accumulated in the accumulator.
As a conventional pump operation control apparatus for controlling the operation of the pump as described above, pump operation control apparatuses have been disclosed in Japanese Patent Laid-Open No. 4-151364 and Japanese Patent Laid-Open No. 3-213701. According to the disclosure, a pump-operating pressure sensor (or a pressure sensor which is hereinafter and representatively called a "pressure sensor") is employed to hydraulic pressure accumulated in the accumulator. When the hydraulic pressure accumulated in the accumulator has been raised to a level with which the pump must be operated, the pump is operated to raise the hydraulic pressure accumulated in the accumulator. Moreover, an alarm-issuing pressure sensor is employed to detect and issue an alarm when the hydraulic pressure accumulated in the accumulator has abnormally been reduced. Moreover, each of the pump operation control apparatuses disclosed as described above has a structure that when a failure of the pump-operating pressure sensor has occurred, the alarm-issuing pressure sensor controls the operation of the pump.
The pump operation control apparatuses disclosed as described above enable predetermined hydraulic pressure to always be accumulated in the accumulator. Even if a failure of the pump-operating pressure sensor has occurred, hydraulic pressure having at least a minimum level required to operate the brake can furthermore reliably be accumulated in the accumulator.
As another conventional pump operation control apparatus, a pump operation control apparatus has been disclosed in Japanese Patent Laid-Open No. 8-58562. According to the foregoing disclosure, depression of a brake pedal is detected by a brake switch. Moreover, time for which the brake pedal is depressed is integrated. When the integrated time has reached a predetermined value, the pump is operated so that hydraulic pressure is accumulated in the accumulator. The pump operation control apparatus disclosed as described above does not require a pump-operating pressure sensor. When a brake switch provided for operating a brake lamp or the like is simply used, the operation of the pump can be controlled to accumulate hydraulic pressure in the accumulator. Thus, the structure of the brake boosting system can be simplified.
However, the above-mentioned pump operation control apparatus disclosed in Japanese Patent Laid-Open No. 4-151364 has the structure that the pump is operated in only a period until the alarm-issuing pressure sensor is turned off at which the alarm-issuing pressure sensor does not transmit the alarm signal if the pump is operated by the alarm-issuing pressure sensor because of a failure of the pump-operating pressure sensor. Therefore, hydraulic pressure cannot sufficiently be accumulated in the accumulator.
If the characteristics of the pump-operating pressure sensor deteriorates, the pressure at which the pump is interrupted is undesirably changed. Thus, the pressure accumulated in the accumulator becomes insufficient, causing the braking force to be reduced excessively. As an alternative to this, the hydraulic pressure accumulated in the accumulator is raised excessively, thus causing the loads which must be borne by the pump, the motor, the accumulator and the like to be enlarged excessively. As a result, there is apprehension that the durability of the above-mentioned elements deteriorates.
The above-mentioned pump operation control apparatus disclosed in Japanese Patent Laid-Open No. 3-213701 has the structure that the pump is operated for predetermined time from a moment at which the alarm-issuing pressure sensor has been turned on in a case where the operation of the pump is controlled by the alarm-issuing pressure sensor because of a failure of the pump-operating pressure sensor. Therefore, if a pressure at which the sensor is turned on is changed because of deterioration of the alarm-issuing pressure sensor or the like, hydraulic pressure cannot sufficiently be accumulated in the accumulator.
The above-mentioned pump operation control apparatus disclosed in Japanese Patent Laid-Open No. 8-58562 has the structure that the time obtained by integrating the time for which the brake pedal is depressed is employed as a parameter for controlling the operation of the pump. Therefore, if the pressure accumulated in the accumulator is reduced in a case where depression of the brake pedal is not performed for a long time because, for example, the vehicle has been allowed to stand for a long time, the pump is not operated. Thus, there is apprehension that the braking force becomes too small.
As described above, both of the pump operation control apparatus disclosed as described above have a problem in that hydraulic pressure cannot always, sufficiently and reliably be accumulated in the accumulator.